Origin

The number ten goes back to an ancient root shared by Latin decem, the source of decimal (early 17th century) and similar words. The rules of conduct given by God to Moses on Mount Sinai, according to the biblical book of Exodus, have been known in English as the Ten Commandments since the Middle Ages. The common practice in schools of setting tests with ten questions has led to ten out of ten coming to mean ‘completely right, perfect’.